Yeah, processing in Photoshop, especially with the channel mixer set to monochrome, really makes a lot of difference.
I would also advise against using images with gradients, as they tend to produce a not-so-pretty banding effect on a 16-colour greyscale screen.
I found that vector illustrator art worked really well in the handful of Hitchhiker screensaver shots I created. Really popped off the screen.
Attached one of the best looking ones I adapted:
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